⏫ Forwardport of #12730 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12730.patch (created by @EfremovaVI) based on commit(s):
  1. 370e526
  2. 5e43f84
  3. ff7029f

Fixed GitHub Issues in 2.3-develop branch:
  - #12713: Currency symbol overlaps entered attribute option's price while creating Configurable Product (reported by @alena-marchenko)

app/design/adminhtml/Magento/backend/Magento_ConfigurableProduct/web/css/source/module/components/_currency-addon.less

@@ -10,7 +10,8 @@
 @currency-addon-symbol__border-color: @field-control__border-color;
 @currency-addon-symbol__color: @color-gray52;
 @currency-addon-symbol__height: @field-control__height;
-@currency-addon-symbol__width: 1.6rem;
+@currency-addon-symbol__width: 2.6rem;
+
 
 //
 //  Common